The invention relates to a method for deciding on the presence of a predetermined pattern at at least one position in a spatial field of data points in which field each data point has a particular amplitude value, the method comprising a comparison between first and second ranked values, the first ranked value having a first predetermined rank in a ranking according to the magnitude of a first collection of amplitude values of data points from a first set of data points at first predetermined locations relative to the position in the spatial field, the second ranked value having a second predetermined rank in a second ranking according to the magnitude of a second collection of amplitude values of data points from a second set of data points at the second predetermined locations relative to the position in the spatial field. The invention also relates to an apparatus for deciding on the presence of a predetermined pattern at at least one position in a spatial field of data points in which field each data point has a particular amplitude value, the apparatus comprising logical decision circuitry for producing a decision from a comparison of amplitude signals at inputs of the circuitry, the amplitude signals being coupled to the circuitry from rank value filters, which filters each have a filter input for receiving a spatial field signal representing the particular values of the data points and producing a ranked amplitude value that occurs at a predetermined rank in a collection of amplitude values of data points from a set of data points at predetermined locations relative to the positin in the spatial field.
A method of this kind is known from European Patent Application No. 89 202 124.7 a foreign counterpart patent to U.S. Pat. No. 5,054,095. The known method detects patterns in which the amplitude values are high in a first set of points and low in a second set of points. For example, in a camera image by way of field of data points, such a pattern would be a certain combination of light areas (the first set) and dark areas (the second set). In the known method, a pattern is said to be detected if all the amplitudes in the first set are above a certain threshold value, and all the amplitudes in the second set are below this threshold. The problem is to decide whether there exists such a threshold. For this purpose, the known method considers the two sets positioned at a predetermined relative position in the field. Subsequently, a lowest boundary value, L, of the range of amplitude values found in the first set is determined, and a highest boundary value, H, is determined for the second set. When L is larger than H, there exists a threshold value as described above, and the pattern is detected.
The known apparatus executing the known method employs a ranked value. A ranked value, R, at a rank n in a collection of mutually different amplitudes, is an amplitude from that collection such that there are exactly (n-1) amplitudes in the collection larger than R. (When the amplitudes are not necessarily mutually different, this definition becomes more complicated: there must be fewer than n amplitudes larger than R in the collection and at least n amplitudes not smaller).
A filter selecting a value at rank 1 produces the highest amplitude value; a filter selecting the amplitude value at rank, N, where N is the number of amplitude values in the collection produces the lowest amplitude value. The reason for using rank filters is that such filters can be effectively implemented in several embodiments even without explicit extraction of the collections of amplitudes. The resulting values are fed into a comparator, in the known apparatus, for forming a detection signal if L exceeds H. Alternatively, the comparator may reserve the detection signal to cases where L exceeds H by more than a predetermined amount.
Although the prior art method and apparatus perform well, they are restricted to pattern detection problems of a substantially binary nature, where there are two sets, one with amplitudes that are high, and one with amplitudes which are low, where the only problem is to decide whether there exists an acceptable threshold. It is therefore an object of the invention to extend the potential of such a method and apparatus, thereby permitting a more differentiated approach to pattern processing.